Mobile communication device such as cellular phones and portable information terminal equipment are widely spread with the developments in the information-oriented society. For example, the cellular phones use RF bands as high as 800 MHz˜1.0 GHz and 1.5 GHz˜2.0 GHz. In order to cope with such RF bands, duplexers using acoustic waves such as acoustic wave resonators or piezoelectric thin-film resonators are used.
Japanese Patent Application Publication No. 2006-60747 (Document 1) discloses an art of using a shield electrode arranged between a transmission filter pattern and a reception filter pattern to improve the isolation between the patterns. Japanese Patent Application Publication No. 2002-76829 (Document 2) discloses an art of arranging external input terminals and external output terminals of a device having multiple filter elements so as to diagonally connect corners of the shape of the device in order to prevent signals from being interfered with each other.
Theoretically, an improvement in the isolation between the transmission and reception terminals of the duplexer may be achieved by designing the transmission filter to have an increased amount of attenuation at frequencies located in the passband of the reception filter and designing the reception filter to have an increased amount of attenuation at frequencies located in the passband of the transmission filter. However, actually, the isolation is affected by not only currents but also current that flow through a space due to electromagnetic coupling. Thus, the isolation is saturated at a certain level.
The influence of the electromagnetic coupling may be reduced by providing an electrode between the filters to realize electromagnetic shield structure as described in Document 1. Alternatively, the physical distance between the transmission and reception terminals may be increased, as described in Document 2. However, the arts disclosed in Documents 1 and 2 may not reduce the influence of the electromagnetic coupling considerably in the downsizing trend of RF communication devices.